Almost 400 people in 42 states have become sick from a salmonella Typhimurium outbreak that has been spreading around the country for a few months.

USA Today reported that officials from the Centers for Disease Control and Prevention (CDC) are having a hard time finding the source of the outbreak, and they have called in experts from other departments to help with the investigation.

The Department of Agriculture, the Food and Drug Administration and state health officials are all working to determine where the salmonella is coming from, according to Reuters. The CDC stated that poultry, cheese and eggs are the primary sources for the Typhimurium strain, which has sent about 70 people to the hospital.

Finding the cause of a food-borne illness can be tough, CDC spokesman David Daigle told Reuters. “People may not remember the foods they recently ate and may not be aware of all of the ingredients in food. That's what makes these types of investigations very difficult.”

Ohio has the second-highest number of people affected by this outbreak, but Kristopher Weiss, a spokesman for the Ohio Department of Health, told Bloomberg he didn’t know which state had more cases.

There may be one clue to the outbreak: Minnesota public health officials warned the public about a jar of peanut butter contaminated with salmonella. The Salmonella was found in a King Nut jar of peanut butter, in a large size that’s distributed to “long-term care facilities, hospitals, schools, universities, restaurants, cafeterias and bakeries,” Reuters said.

It’s not yet known whether that brand of peanut butter is available in grocery stores, and officials have not yet said whether the peanut butter is responsible for the current outbreak. But a state health department statement Reuters quoted said, “The Minnesota cases have the same genetic fingerprint as the cases in a national outbreak that has sickened almost 400 people in 42 states.”

From April through August 2008, health officials conducted one of their largest investigations into food-borne illnesses in a decade when a salmonella outbreak sickened hundreds of people around the country. Raw red round, red Roma and red plum tomatoes were originally cited as potential causes of the outbreak, but the investigation later turned to shipments of peppers and chilies from Mexico. When the investigation centered on tomatoes, millions of dollars in merchandise was destroyed.

In 2008, federal health officials said food-borne illnesses had remained constant in the United States since 2004. Food safety experts maintained that people were more likely to be involved in a car crash than become ill from eating vegetables. “Fear is still by far the biggest pathogen here, not salmonella,” columnist Marc Siegel was quoted as saying.

People infected with salmonella will likely experience fever, diarrhea and abdominal cramps between 12 and 72 hours after infection. Generally, treatment is not required, but in cases where diarrhea is too severe, or the infection spreads from the intestines, antibiotics or IV fluids may be necessary.

Consumers can protect themselves from contracting salmonella by thoroughly cooking meats, poultry and eggs to destroy microorganisms. Avoiding cross-contamination is also helpful. If you work with a food that can contain salmonella, wash your hands and clean your cooking surfaces and utensils before handling another food product. People should wash their hands after playing with pets because animals can contract salmonella as well.